1. Field of Invention
The present invention relates to a communication antenna. More particularly, the present invention relates to an antenna with an additional capacitor, so as to reduce the dimension of the antenna and maintain the required LC coupling strength.
2. Description of Related Art
The wireless communication system always needs an antenna to transmit and receive RF signals. In recent years, the wireless communication technology has been well developed. For example, the cellular phone is one of the impressing apparatus in wireless communication. The dimension of the cellular phone require an antenna. In order to implement the antenna in compact space, a planar, a line inverted-F, or L-type antennas have been proposed. However, these conventional antennas are not implemented on the plane of a printed circuit board (PCB). Also the antenna has to match to a certain ratio of the wavelength, such as 1/4 λ, with respect to the transmission frequency of, i.e. about 2.4 GHz.
FIG. 1 shows a transmission line of characteristic impedance Z0, propagation constant β, and length L, loaded with an impedance ZL. By the transmission line theory, the input impedance Zin and the corresponding input admittance Yin can be expressed as follows:                                           Z                          i              ⁢                                                          ⁢              n                                =                                    Z              o                        ⁢                                                            Z                  L                                +                                  j                  ⁢                                                                          ⁢                                      Z                    o                                    ⁢                  tan                  ⁢                                                                          ⁢                  β                  ⁢                                                                          ⁢                  L                                                                              Z                  o                                +                                  j                  ⁢                                                                          ⁢                                      Z                    L                                    ⁢                  tan                  ⁢                                                                          ⁢                  β                  ⁢                                                                          ⁢                  L                                                                    ,                            (        1        )                        and                      (        2        )                                                      Y                          i              ⁢                                                          ⁢              n                                =                                    Y              o                        ⁢                                                            Y                  L                                +                                  j                  ⁢                                                                          ⁢                                      Y                    o                                    ⁢                  tan                  ⁢                                                                          ⁢                  β                  ⁢                                                                          ⁢                  L                                                                              Y                  o                                +                                  j                  ⁢                                                                          ⁢                                      Y                    L                                    ⁢                  tan                  ⁢                                                                          ⁢                  β                  ⁢                                                                          ⁢                  L                                                                    ,                                                            where        ,                              Y                          i              ⁢                                                          ⁢              n                                =                      1                          Z                              i                ⁢                                                                  ⁢                n                                                    ,                              Y            o                    =                      1                          Z              o                                      ,                              and            ⁢                                                  ⁢                          Y              L                                =                                    1                              Z                L                                      .                                                          If ZL is zero (short-circuited), the input impedance Zin, denoted by Zins, isZins=jZo tan βLorYins=−jYo cot βL.If ZL is infinite (open-circuited), the input impedance Zin, denoted by Zino, is,Zino=−jZo cot βLorYino=jYo tan βL.
Based on the antenna theory, FIG. 2 shows an in conventional design of L-type compact antenna. In FIG. 2, the L-type antenna 102 includes a tuning section 102a and a signal feeding section 102b connected at one end by a right angle, wherein a signal feeding line 103, which is the portion over the ground layer 100, is coupled to the signal feeding section 102b. A ground layer 100 is implemented under a signal feeding line 103 without direct connection. The tuning section 102a itself provides the LC coupling with the ground layer. An insulating layer (not shown) may exist between the ground layer 100 and the signal feeding line 103. The insulation layer in the specification is omitted but can be understood by the skilled artisans.
FIG. 3 shows the antenna mechanism to the L-type antenna in FIG. 2. In conventional design, the L type antenna is based on concept of monopole antenna. The length of tuning section 102a in monopole approximates quarter wavelength in resonant frequency. The tuning section 102a and the nearby ground plane 100 form an open-ended transmission line. The input impedance of this open line is Zin=−jZo cot βL′, which corresponds to an equivalent capacitance CM of       C    M    =                    tan        ⁢                                  ⁢        β        ⁢                                  ⁢                  L          ′                            ω        ·                  Z          0                      .  The equivalent capacitance would resonate at the angular frequency ω with the small inductance provided by the signal feeding section 102b of the L-type antenna.
Another type of conventional antenna is an inverted F antenna as shown in FIG. 4. The antenna 200 includes the short circuit stub section 200a, a signal feeding section 200c, and a tuning section 200b, which elements couple together at a joint. The inverted F antenna is similar to the L-type antenna but additionally includes the short circuit stub section 200a, which is directly coupled to the ground layer 100. FIG. 5 shows the antenna mechanism with respect to the inverted F antenna shown in FIG. 4.
In the foregoing conventional antennas, the tuning section 200b is a straight line and has a required length to satisfy the receiving/transmission operation with respect to the working frequency. Usually, the length L is 1/4 λ to have sufficient LC coupling effect. This causes the dimension to be large.
Moreover, the conventional antenna is implemented, extending outward on the house of the communication apparatus. This is not a compact design, and needs additional fabrication process.